A radio frequency power amplifier is a type of electronic amplifier used to convert a low-power radio-frequency signal into a larger signal of significant power, typically for driving the antenna of a transmitter. It is typically optimized to have high efficiency, high output Power (P1dB) compression, good return loss on the input and output, good gain, and optimum heat dissipation.
A radio transmitting in the downlink in a communication system is typically designed to deliver a pre-defined power level to an antenna, which is in turn designed to radiate in a certain direction to some User Equipment (UE). In doing so, it is at the same time usually required that the signal has a certain quality, and also, in a system view, required that leakage in adjacent channels/frequencies (spectrum, or spatial) is complying with a certain value, either specified by corresponding standard or sometimes by an official regulation.
The unwanted signal emissions, i.e. the leakage in adjacent channels/frequencies, arise from components in the transmission chain being more or less non-linear, thus causing spectrum broadening. The level of this spectrum broadening may be directly related to the signal quality, regardless of whether the signal is being used as a “wanted signal” or if the signal is being viewed as an interfering signal, i.e. unwanted emissions, to some kind of victim system.
A way to combat these kinds of imperfections, i.e. non-linearities, and ensure that the required level of unwanted emission is kept below a certain limit or value, is to use an analog or digital linearizer to pre-distort the signal in a proper way, so that the unwanted emissions caused by the non-linearity, e.g. of a power amplifier, are canceled in the transmission link. A predistortion circuit inversely models the amplifier's gain and phase characteristics and, when combined with the amplifier, produces an overall system that is more linear and reduces the amplifier's distortion. In essence, “inverse distortion” is introduced into the input of the amplifier, thereby cancelling non-linearities of the amplifier. As radio power amplifiers tend to become more non-linear as their output power increases towards their maximum rated output, predistortion is a way to get more usable power from the amplifier, without having to build a larger, less efficient and more expensive amplifier. Linearization, or predistortion, is a very common approach today. This method is also applied in systems having two or more transmission paths, i.e. radios, in parallel, such as in a so-called active antenna array, or Active Array System (AAS).
However, as the number of active transceivers branches, i.e. transmission paths/radios, increase in an AAS, the complexity of linearizing all of the transmission paths grows dramatically. It is clear that when coming above a certain number of transceiver branches, the processing power and capacity used for the linearizers exceeds the processing power and capacity used for other functionality, and thereby traffic capacity, as well as power efficiency can be severely degraded. Further, as the number of active transceiver branches in an AAS increases, the hardware cost and complexity associated with the linearization becomes a problem.